What you're missing is that the first parameter to grep is supposed to be an expression or a block of code, whose return value determines success or failure of each match. In your case, that return value is found by evaluating $username, which is most likely a true value. So then the grep gives you every single element from @ignore because the expression was true for every one of them. What you really meant to do is either a straight comparison:

grep $username eq $_, @ignore
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or a pattern match:

grep /$username/, @ignore
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Note that the second is still an expression, and gets evaluated with each element in @ignore aliased to $_ in turn. You can just write it in this short form because by default, matches are attempted against $_ unless otherwise specified. So the second form is exactly equivalent to this:

grep $_ =~ /$username/, @ignore
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Makeshifts last the longest.